1. Field of the Invention
The present invention is generally related to shipbuilding and particularly to the joining of separate modules in building large ships.
2. General Background
For centuries a typical large wooden ship was constructed on a building ways under the supervision of a master shipwright. The wooden parts were fabricated piece by piece, and then fastened together on the building ways.
Shipbuilding techniques that proved effective in constructing wooden ships were often retained when wood was replaced, first by iron and later by steel. The shipyard's building ways was still the place where a highly skilled work force assembled the ship, piece by piece. The keel was laid first, followed by floors, frames, beams, decks, and hull plating. Initially metal structural members were joined together by riveting.
Since about 1940 the construction of both commercial and naval vessels has changed dramatically. These changes may, in part, be attributed to the development of new steels, the use of welded instead of riveted connections, the increased use of aluminum, the advent of computer-aided technology, construction orders for several identical ships, and, above all, the need to improve productivity. In conjunction with economic pressures, these changes have forced shipyards into developing a more efficient assembly process--one in which the flow of material, the building of subassemblies, and the final fabrication of the ship are merged to form a continuous process.
A modern shipyard is predominately an assembly facility. Subassemblies are joined together to form still larger modules, which in turn form major sections of the hull. These modules are then moved by large overhead cranes to various outfitting areas, where machinery foundations, pipes and valves, ventilation trunks, electrical cables, and various pieces of machinery are installed.
After as much of the outfitting as possible has been completed, the modules, typically weighing several hundred tons, are moved by gantry crane to the building dock or final assembly area. Here the modules are joined ("spliced") together to form a nearly complete ship. The long seams are then welded together mechanically.
Depending on the shipyard's arrangement and physical facilities, the actual launching may be performed by a variety of methods. A side or end launch may be performed from a building ways. The ship may also be launched from either a floating dry dock or a building dock, by flooding the dock until the ship floats. Building docks are very expensive and generally are used only for the final assembly of a ship's previously fabricated modules.
The key point is that in traditional shipbuilding, all large components are joined together while resting on or above dry land, with large, expensive dry or building docks needed to complete final assembly.
An efficient method for making the final, joining splice(s) in a floating position would eliminate the very expensive graying docks needed to construct mid-size ocean vessels.
Patents directed to modular shipbuilding or repair which applicant is aware of include the following.
U.S. Pat. Nos. 5,090,351 and 5,085,161 are directed to the construction of double-hulled tankers. A pontoon caisson provides an evacuated area for above and below-the-butt joint preparation and welding. The pontoon caisson is stationary and does not have its buoyancy adjusted. The hull modules are partially flooded to assist tilting of the modules by a derrick and movement of the modules over the caisson. Once over the caisson, the modules are flooded to submerge them hard aground on the caisson for joining.
U.S. Pat. No. 3,370,565 discloses a method and apparatus used to cut or join a structure floating on the water. A band is positioned over the area of the ship to be worked upon. The band is held in position by cables and sealed against the ship by pressurized fluid in grooves provided in the band. A working space in the center of the band that is positioned over the ship area to be worked upon is drained of water by making borings inside the hull. The working space is provided to vent gas produced during welding or cutting.
U.S. Pat. No. 4,155,322 discloses a floating caisson device to be placed below a vessel hull to provide a working space when joining floating hull sections. A flexible transition member on each end of the caisson that does not touch the welding zone on the hull is used to pull a packing member on each side of the hull into sealing engagement along the side length of the hull.
U.S. Pat. No. 3,611,968 discloses a watertight floating box with at least one sidewall being movable to accommodate different width vessels. Packings are pulled into contact with the vessel on either side of the welding area. Compressed air is introduced into the packings to establish a seal with the vessel.
U.S. Pat. No. 3,585,954 discloses a watertight floating caisson box having a movable side member to accommodate different width vessels. Watertight packings contact the vessel modules on either side of the welding area. This provides a dry work area once water has been removed from the area between the packings.
U.S. Pat. No. 3,407,771 discloses the use of holding plates attached to the exterior of shell plates on separate hull parts. The two hull parts are floated together such that the holding plates abut in a watertight engagement. Water in the adjoining hull parts is removed and the hull parts are then welded together.
U.S. Pat. No. 4,686,919 discloses a method for constructing large marine structures. The blocks of the marine structure are constructed in a deck and towed to an adjacent pond. Gates enclosing the pond are closed and the blocks are tack welded together. Water is drained from the pond to ground the tack welded blocks and the blocks are then fully welded together.